Dispersed and partially purified human pulmonary mast cells are being assessed for the morphologic and biochemical concomitants of degranulation and release of mediators of immediate hypersensitivity. The de novo generation of slow reacting substance of anaphylaxis (SRS-A) and oxidative metabolites of arachidonic acid by IgE-dependent mechanisms is under investigation. Efforts to purify and chemically characterize SRS-A continue.